JPH04362155A - High purity steel for integrated high and low pressure turbine rotor - Google Patents

Abstract

PURPOSE:To provide high purity steel for the integrated high and low pressure rotor of a turbine usable for a large-sized rotor because of enhanced strength and toughness. CONSTITUTION:This high purity steel consists of, by weight, 0.2-0.35% C, 1.6-2.4% Bi, 1.2-2.5% Cr, 0.9-1.5% Mo, 0.2-0.3% V and the balance Fe with inevitable impurities or further contains 0.01-0.05% Nb. The allowed contents of Si, Mn, P and S among the inevitable impurities are <=0.1%, <=0.1%, <=0.005% and <=0.005%, respectively.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to high-purity steel for high-low pressure integrated turbine rotors used for turbine rotor shafts of power generators.

0002

2. Description of the Related Art As is well known, there are various types of turbine generators, one of which is a single-casing turbine generator that uses an integrated high and low pressure turbine rotor. This turbine rotor is exposed to steam pressure ranging from high to low pressure at high temperatures, and its materials include:
It is required to have both excellent high-temperature creep properties and excellent low-temperature toughness. Conventionally, Cr-Mo-V low alloy steels have been developed as materials for high and low pressure integrated turbine rotors.
No. 370 discloses a low alloy steel that is an improved version of this type of material. By the way, the above-mentioned high and low pressure integrated turbine rotor has traditionally been a small one with a body diameter of about 1 m, but in order to improve energy efficiency, a large high-speed rotor with a body diameter of 2 m has been used. The development of a low-pressure integrated turbine rotor is desired.

0003

[Problem to be solved by the invention] In order to realize a larger high-low pressure integrated turbine rotor, higher strength and toughness of the material is required compared to conventional small rotors, and It is difficult to meet this demand with materials. For example, when trying to apply Cr-Mo-V low alloy steel, which has been conventionally developed as a material for small high-low pressure integrated turbine rotors, to large high-low pressure integrated turbine rotors, it lacks high-temperature creep strength or low-temperature toughness. Sufficient performance cannot be obtained as a large high/low pressure integrated turbine rotor material.

The present invention was made against the background of the above circumstances, and an object of the present invention is to provide a high-purity steel for a high-low pressure integrated turbine rotor that has both excellent high-temperature creep properties and excellent low-temperature toughness. It is something.

[0005]

[Means for Solving the Problems] In order to solve the above problems, the first invention of the high purity steel for high and low pressure integrated turbine rotor of the present invention has a carbon content of 0.2 to 0.35 in weight%. %
, Ni: 1.6-2.4%, Cr: 1.2-2.5
%, Mo: 0.9-1.5%, V: 0.2-0.3%
with the remainder consisting of Fe and unavoidable impurities,
Among the inevitable impurities, in weight percent, Si: 0.1% or less, Mn: 0.1% or less, P: 0.005% or less, S
: The permissible content is 0.005% or less.

The second invention is characterized in that the composition of the first invention further contains Nb: 0.01 to 0.05% by weight.

[0007] The unavoidable impurities in the first and second inventions above are not limited to components whose content is limited, but also include impurities that are usually unavoidably mixed. There are no particular numerical limitations on these components.

[0008]

[Action] According to the high purity steel for high and low pressure integrated turbine rotors of the present invention, the hardenability is improved, and through heat treatment, even large rotors are sufficiently hardened down to the center of the large body diameter, resulting in high temperature creep. Low-temperature toughness can be improved without impairing strength. Furthermore, by suppressing the content of unavoidable impurities and achieving high purity, susceptibility to temper embrittlement is improved and aging deterioration can be suppressed.

Next, the reason for limiting the component content of the present invention will be described below. In addition, in the following description, the content of each component is shown in weight%. C: 0.2-0.35% In order to obtain the desired tensile strength and yield strength, C content must be 0.2% or more, but if it exceeds 0.35%, the toughness decreases, In addition, since carbide agglomeration and coarsening occur, lowering the creep strength, the above range is set. Ni: 1.6 to 2.4% Ni is added to improve hardenability, strength, and toughness. However, if the content is less than 1.6%, its effect is insufficient, and if it is more than 2.4%, the high-temperature creep strength is reduced, so the range was set as above.

Cr: 1.2 to 2.5% added to improve high temperature strength and toughness. However, the content is 1.
If it is less than 2%, its effect will be insufficient;
Even if the content exceeds %, the effect will be saturated, so the above range was set. Mo: 0.9 to 1.5% Forms carbides with C and finely precipitates in the matrix,
Improves strength at low and high temperatures, and further suppresses temper embrittlement. If the content is less than 0.9%, the effect will be insufficient, and if the content exceeds 1.5%, the effect will not only be saturated, but also the high temperature strength and toughness will be reduced, so the above range is required. And so.

V: 0.2 to 0.3% V forms carbides and improves high temperature strength. however,
If the content is less than 0.2%, its effect is insufficient, and if it is contained in more than 0.3%, the high temperature creep strength
Since it lowers toughness, it is set in the above range. Nb: 0.01 to 0.05% Nb forms carbide and increases high temperature strength, so it is added as desired. However, if the content is less than 0.01%, the effect is insufficient, and if the content exceeds 0.05%, eutectic carbides are formed and the toughness is significantly deteriorated, so it is limited to the above range. .

Unavoidable impurities (Si: 0.1% or less, M
n: 0.1% or less, P: 0.005% or less, S: 0.
0.05% or less) Si, Mn: 0.1% or less Si is usually used as a deoxidizing agent, and in that case the content is:
It is usually about 0.30 to 0.50%. This level of Si
, macro-segregation occurs in large steel ingots. Moreover, when the Si content is high, the susceptibility to temper embrittlement becomes extremely high, and notch toughness is impaired. In the present invention, in order to avoid the above-mentioned adverse effects of Si, for example, Si
Vacuum C deoxidation is used instead of deoxidation. When performing vacuum C deoxidation, it is desirable to reduce the Si content as much as possible before deoxidation, and in the present invention, Si as an unavoidable impurity is
The permissible content of is limited to 0.1% or less, which is industrially possible. Mn combines with S to form nonmetallic inclusions,
Decreases toughness. In addition, Mn remaining in the steel is S
Like i, Mn promotes temper embrittlement, so it is desirable to reduce it as much as possible, and the allowable content of Mn as an unavoidable impurity was limited to 0.1% or less, which is industrially possible. .

P: 0.005% or less P is an element that promotes susceptibility to temper embrittlement, and in order to obtain materials with little aging deterioration, it is desirable to reduce it as much as possible, taking into consideration the current level of refining technology. Therefore, the allowable P content was limited to 0.005% or less. S: 0.005% or less In large steel ingots, even a trace amount of S will generate nonmetallic inclusions such as Mn and S in the steel, degrading the quality of the steel, so it is desirable to reduce it as much as possible. Similarly to P, considering the current refining technology level, the allowable content of S is set to 0.00.
It was limited to 5% or less.

Other Unavoidable Impurities In addition to the above-mentioned unavoidable impurities, unavoidable impurities that deteriorate steel quality include Cu, and unavoidable impurities that promote temper embrittlement include As, Sb, Sn, etc. It is preferable to reduce these unavoidable impurities as much as possible. However, these unavoidable impurities are unavoidably mixed in with the raw materials and are difficult to remove by refining. Therefore, it depends largely on the careful selection of raw materials, and from the perspective of improving steel quality, Cu: 0.10
% or less, As: 0.008% or less, Sb: 0.01
It is desirable to limit Sn to 0.005% or less.

[0015]

[Example] The present invention steel and comparative steel having the compositions shown in Table 1 were melted in a vacuum melting furnace to produce a 50 kg steel ingot. 1 piece of each steel ingot
It was heated to 200°C, hot forged at a forging ratio of about 4, further heated to 1050°C, quenched, and then sample steel No.
．． 1 to 5 and No. 7.No. 8 is 20 at 650℃
Subjected to time tempering. In addition, test steel No. No. 6 was obtained by changing the tempering temperature. 6A is 650°C, No. 6B is 6
55°C, No. 6C was tempered at 660°C for 20 hours.

Next, Table 2 shows the material test results of the sample steel after heat treatment. Based on the results in Table 2, the relationship between fracture surface transition temperature and creep rupture time is illustrated in FIG. As is clear from Figure 1, compared to the comparison steel, the fracture surface transition temperature of the steel of the present invention is lower, the creep rupture time is also longer, and both high-temperature creep strength and low-temperature toughness are It also has excellent performance and can be applied to large high and low pressure integrated turbine rotors.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

Effects of the Invention As explained above, according to the high-purity steel for high-low pressure integrated turbine rotor of the present invention, C
:0.2~0.35%, Ni:1.6~2.4%, C
r: 1.2-2.5%, Mo: 0.9-1.5%,
Contains V: 0.2 to 0.3%, the remainder consists of Fe and unavoidable impurities, and among the unavoidable impurities, Si: 0.1% or less, Mn: 0.1% or less, P :
Since the permissible content is 0.005% or less and S: 0.005% or less, hardenability is improved, and even large rotors can be sufficiently hardened, and low-temperature toughness is improved without compromising high-temperature creep strength. can be done. Furthermore, the susceptibility to temper embrittlement is improved and deterioration over time can be suppressed. Therefore, when applied to a high/low pressure integrated turbine rotor, the rotor can be made larger and energy efficiency can be improved.

[0020] Furthermore, by containing Nb: 0.01 to 0.05%, in addition to the above effects, there is an effect of further increasing high temperature creep strength.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between creep strength and low temperature toughness of test steel.

Claims (2)

[Claims] Claim 1: C: 0.2 to 0.35% by weight,
Ni: 1.6-2.4%, Cr: 1.2-2.5%
, Mo: 0.9 to 1.5%, V: 0.2 to 0.3%, the remainder consisting of Fe and unavoidable impurities, and among the unavoidable impurities, Si: 0.1% by weight. % or less, Mn: 0.1% or less, P: 0.005% or less, S:
High purity steel for high and low pressure integrated turbine rotor, characterized by an allowable content of 0.005% or less 2. A high-purity steel for a high-low pressure integrated turbine rotor, characterized in that the composition according to claim 1 further contains Nb: 0.01 to 0.05% by weight.